Maskasky U.S. Pat. No. 5,292,632, titled HIGH TABULARITY HIGH CHLORIDE EMULSIONS WITH INHERENTLY STABLE GRAIN FACES, and Maskasky U.S. Pat. No. 5,264,337, titled MODERATE ASPECT RATIO TABULAR GRAIN HIGH CHLORIDE EMULSIONS WITH INHERENTLY TABLE GRAIN FACES, each commonly assigned, each disclose the preparation of high chloride tabular grain emulsions in which the tabular grains have {100} major faces, hereinafter referred to as high chloride {100} tabular grain emulsions. Unless otherwise qualified subsequent references to "Maskasky" refer to these two filings as collective entity. The technique which Maskasky employs to cause tabular grains to form is to employ during grain nucleation and growth a restraining agent to prevent the emergence of non-{100} grain faces. The restraining agents disclosed are each organic compounds containing a nitrogen atom with a resonance stabilized .pi. electron pair. The trivalent nitrogen atom is either directly bonded to an aromatic ring, as illustrated by aniline, or present in the ring, as illustrated by indole, pyridine and 1,3,5-triazine.
Although Maskasky was able to produce high chloride {100} tabular grain emulsions, the requirement of an organic restraining agent adsorbed to the faces of the tabular grains is a disadvantage. Many components of photographic emulsions, such as spectral sensitizing dyes, antifoggants and stabilizers, latent image keeping addenda and nucleating agents require adsorption to grain surfaces to be effective. To the extent that these photographically useful addenda must compete with the restraining agent for grain surface access the photographic effectiveness of the emulsions are diminished.
House et al U.S. Pat. No. 5,320,938, titled HIGH CHLORIDE TABULAR GRAIN EMULSIONS AND PROCESSES FOR THEIR PREPARATION, commonly assigned, discloses a process for the preparation of high chloride {100} tabular grain emulsions that runs contrary to the teachings of Maskasky and other, earlier high chloride tabular grain emulsion preparation teachings. Instead of employing an adsorbed organic restraining agent to cause the tabular grains to form House et al relies upon the presence of iodide ion at the grain nucleation site to form improved high chloride {100} tabular grain emulsions.
Maskasky U.S. Pat. No. 5,292,632 requires at least 30 percent of total grain projected area to be accounted for by high chloride {100} tabular grains, while many of the Examples produce emulsions in which the tabular grains account for less than 50 percent of total grain projected area. Maskasky U.S. Pat. No. 5,264,337 and House et al require the high chloride {100} tabular grains to account for at least 50 percent of total grain projected area, while many of the Examples produce emulsions in which the tabular grains account for less than 80 percent of total grain projected area. Maskasky and House et al each disclose that the high chloride {100} tabular grains can account for greater than 90 percent of total grain projected area, but no actual measurement of a projected area in excess of 90 percent is provided. House et al reports two different emulsions in which high chloride {100} tabular grains account for nominally greater than 90 percent tabular grains; however, review of these precipitations shows that in no instance did the nominally "greater than 90 percent projected area" approach a projected area of 95 percent. In fact, each of the emulsions of Maskasky and House et al contain appreciable inclusions of unwanted grain populations in addition to the high chloride {100} tabular grains sought.
Brust et al U.S. Pat. No. 5,314,798, titled IODIDE BANDED TABULAR GRAIN EMULSIONS, commonly assigned, initiating high chloride {100} tabular grain growth by the procedures taught by Maskasky or House et al, followed by the introduction of a higher iodide band, preferably in the latter stages of precipitation. The higher iodide band improves the speed-granularity of the emulsion, but has little, if any, influence on the percentage of total grain projected area accounted for by tabular grains.